“It was surprising to me how much genetic structure there was within a nation as relatively small as the UK,” co-author Garrett Hellenthal says. “Previous work from others has shown genetic differences among different European countries, but it was striking to see we could tell apart neighboring counties within England.”

Researchers began by collecting over 2,000 DNA samples from within the UK. By comparing them against 6,000 other European genetic samples, Donnelly’s team was able to trace the people of Britain back to their ancestral roots. The study accounted only for families living on the Isles for multiple generations, however – beginning in the 20th century, mass immigration has rapidly diversified the population. But while Donnelly's map isn't exactly up-to-date, it does provide insights into the distant cultural background of the UK – and even settles some historical disputes in the process.

Researchers found that England’s population was a mostly homogeneous genetic group – one that was no more than 40 percent Anglo-Saxon. It had been argued that the Anglo-Saxon migrations wiped out indigenous populations, but the DNA tells a different story – that Anglo-Saxons intermarried with, rather than replaced, native people.

Similarly, researchers found that invading Norse Vikings in the 9th century mixed with, and did not eliminate, the native population of Orkney. The international team also found that, of all the groups, the Welsh are the most genetically similar to the earliest settlers of the British Isles, who migrated there sometime after the last ice age.

In another surprising breakthrough, researchers found that there was no single “Celtic” genetic group. Their data seemed to indicate that the Cornish, Irish, Scottish, Welsh, and Manx were notably different from each other – often more closely resembling other English groups.

“The differences between these two may be due to relatively recent isolation, due to people not traveling between the two areas much and intermixing in recent history. An interesting area of future work is to determine whether the DNA differences among these Celtic groups are due entirely to such relatively recent isolation and Anglo-Saxon migrations, suggesting that – without these recent events – indeed they were very genetically similar prior to the last 2000 or so years. But we did not do that with this study.”

And while there may not be a single genetic signature for Celtic people, cultural identity was never really about genetics.

“As for ethno-cultural identity, the fact we observe subtle but real genetic differences between individuals living in neighboring counties suggests that individuals from these areas have been somewhat isolated from each other (or at least have not intermixed) over some recent period of time,” Hellenthal said, “which in turn could mean that – through this period of isolation – they developed divergent cultural practices. However, by no means are we suggesting we've found a genetic basis for any such cultural differences; as far as we know, most of the genetic differences between UK groups don't affect or encode for any physical traits or characteristics.”

The findings reveal how genetics can contribute to our understanding of our past.

“As a statistician I was amazed at how successfully DNA, when you have a lot of it, can tease apart these different historical events,” Hellenthal said. “It shows that genetics can make a great independent complement to other fields, such as archaeology and linguistics, in inferring human history and resolving controversies in those fields.”

[Editor's note:This article was corrected to reflect that the map in question is not representative of the current UK population.]